Effects of Nitrogen Application on Root Length and Grain Yield of Rain‐Fed Maize under Different Soil Types

The effect of fertilizer N on maize ( Zea mays L.) root size has been reported with inconsistency. It remains unclear whether a quantitative relationship exists between soil NO 3 –N and root growth under field conditions. A 3‐yr field experiment was conducted in three soils (loamy clay, clay loam, and sandy loam) with five N treatments (from 0–312 kg N ha −1 ). Soil NO 3 –N concentration and total root length were determined to a depth of 60 cm at the silking stage. It was found that plants grown in clay loam soil had larger root size compared with the other two soil. A weak parabolic relationship between N supply and root length was found in loamy clay and clay loam soil, but not in sandy loam soil. There was a parabolic relationship between soil NO 3 –N concentration in the 0‐ to 60‐cm depth and total root length per plant. This relationship was mostly found in clay loam soil. Across soils and years, there was a parabolic relationship between root length density (RLD) and shoot dry matter (DM) at silking and between RLD at silking and grain yield with fertilizer N rate £240 kg N ha −1 . It is suggested that it may be not practical to regulate total root length by simply adjusting N fertilizer amount. Total root length is not relevant to maize yield under excess N input conditions. Core Ideas Plants grown in clay loam soil had larger root than in loamy clay and sandy loam soils. A parabolic relationship between soil nitrate‐N concentration and total root length. A weak parabola relationship between N supply and root length was found in loamy clay soil and clay loam soil, but not in sandy loam soil. Root length and the maximum yield reached synchronously at optimum N rates (168–240 kg N ha −1 in the present study).